1. Field of Invention
This invention relates to a monitoring circuit and, in particular, to a monitoring circuit for motor vehicle maintenance.
2. Background of the Prior Art
The increased consumer awareness and the increasing concern for safety of products such as automobiles has resulted in the development of various monitoring circuits to detect any potentially hazardous condition. The development of these circuits has also been spurred by the desire to increase the efficiency of routine inspections and maintenance of vehicles.
Examples of recent patents on automotive monitoring systems are U.S. Pat. Nos. 3,852,712 and 3,894,108. The first of these patents employs an indicator drum which bears a plurality of legends identifying the particular, sensed parameter with a warning light to indicate a hazardous condition. The second of these patents discloses a monitoring circuit in which signals from a plurality of vehicle condition sensors are passed into a trip circuit to activate alarm devices.
One of the failings of the prior devices is the failure to monitor their internal circuitry such that an indicator or alarm is activated to warn the user of malfunction of the circuit resulting from an open or shorted circuit lead. Additionally, it is desirable to provide a monitoring circuit which employs a minimum number of discrete components which preferably are obtainable in commercially available integrated circuits. The system should also preferably employ its own battery, independent of the vehicle battery and, for simplification, should comprise a single wire system, utilizing the vehicle chassis for the common ground.